Understanding how heat moves in a room is critical for designing comfortable and energy-efficient buildings. A Radiation & Natural Convection In a Room CFD simulation allows us to see this invisible process. In a typical room, heat is transferred by two main methods: natural convection, where air circulates due to temperature differences, and thermal radiation, where hot surfaces emit heat waves. This report details a CFD analysis that simulates both of these effects, showing how a hot wall and a cold window work together to create airflow and distribute heat within a room.

This type of analysis is a perfect example of the skills taught in our comprehensive ANSYS Fluent course for beginners, a Course that provides an excellent foundation for performing your own thermal simulations.

Figure 1: Natural convection in a room

Simulation process: Modeling Radiation & Natural Convection In a Room Fluent Simulation

The simulation was performed in ANSYS Fluent using a 2D model of a room with one hot wall (representing a heater or a sun-lit wall) and one cold wall (representing a window). To accurately capture the physics of this Natural Convection In a Room CFD problem, several key models were enabled:* The Energy Equation was activated to solve for heat transfer.

• Gravity was turned on, which is essential for buoyancy-driven flow.
• The Boussinesq approximation was used to model the change in air density with temperature. This density change is the engine that drives natural convection.
• A Radiation Model (such as Discrete Ordinates or S2S) was activated to account for heat being transferred directly from the hot wall to all other surfaces in the room.

Post-processing: CFD Analysis, How a Temperature Difference Drives a Circulation Loop

The simulation results provide a clear and fully substantiated story that begins with the temperature difference between the walls, which is the primary “cause” of all motion. The hot wall on the left transfers heat to the air molecules it touches. The immediate “effect” of this heating is that the air becomes less dense and lighter. At the same time, the cold window on the right cools the air next to it, making it denser and heavier. This difference in density creates a powerful, invisible force called buoyancy. This buoyancy is the engine that forces the warm, light air to rise along the hot wall and the cool, dense air to sink along the cold wall. The temperature contour in Figure 2 is the perfect visual proof of this, showing a distinct plume of hot air (red/yellow) rising from the left wall and a plume of cold air (blue) falling from the right wall.

Figure 2: Velocity Contour in Room Cross-Section

This rising and falling air is the “cause” of the next, larger-scale effect: the formation of a single, powerful circulation loop that dominates the entire room. The rising hot air travels across the ceiling, gradually cools, and then sinks down the cold wall. It then travels back across the floor, is reheated by the hot wall (and also by the floor, which has been warmed by direct Radiation Fluent from the hot wall), and completes the cycle. The velocity streamlines in Figure 3 are the definitive proof of this room-scale circulation, showing a clear, large-scale vortex that actively transports heat from the hot side to the cold side of the room. This process continuously mixes the air, trying to create a uniform temperature. The most significant achievement of this analysis is the clear demonstration of how a simple temperature difference (the cause), when combined with the effects of radiation and gravity, naturally organizes itself into a powerful, room-wide convection cell (the effect). This validated CFD model provides engineers with a crucial tool to predict air temperature and velocity profiles, allowing them to optimize heater and window placement for maximum thermal comfort and energy efficiency.

Figure 3: Temperature streamlines from the Radiation & Natural Convection In a Room Fluent simulation, clearly illustrating the large, single convection cell that drives air circulation throughout the space.